Method and apparatus for ion bombardment using negative ions

ABSTRACT

Improved stability and control in ion bombardment is achieved in the case of many materials under bombardment by the use of negative ions. The method is especially advantageous for the analysis of insulating materials by secondary ion emission and also enables improved results in ion implantation processes.

United States Patent Cristian A. Andersen Solvang;

Henry J. Roden, Santa Barbara, Calif. 723,026

Apr. 22, 1968 Apr. 6, 1971 Applied Research Laboratories, Inc. Sunland,Calif.

Inventors Appl. No. Filed Patented Assignee METHOD AND APPARATUS FOR IONBOMBARDMENT USING NEGATIVE IONS 3 Claims, 3 Drawing Figs.

11.s.c1 2s0/41.9, 148/1.5, 250/49.5 1111. C1 H0lj 39/34, 11011 7/00Field of Search l48/l.5;

250/41.9 (ISB), 41.9 (ISE), 49.5 (9) [56] References Cited UNlTED STATESPATENTS 2,947,868 8/1960 l-lerzog 250/49.5(9) 3,328,210 6/1967 McCaldinet a1. 148/1.5 3,336,475 8/ 1967 Kilpatrick 250/43 3,341,754 9/1967Kellett et al. 148/1 .5X

OTHER REFERENCES Zhumal Tekhnicheskoi Fiziki, Vol. 30, N0. 1, Fogel etal., pgs. 63- 73, January, 1960, 250 49.5(9)

Primary ExaminerJames W. Lawrence Assistant Examiner-A. L. BirchAttorneyHoffman Stone ION SOURCE FOCUSSING MASS SPECTRO- JLK METERSPECIMEN PMented A ril a 1m 395mm 5'0 K50 5'0 b0 60 2B0 SECONDS SECONDSFIG. 1 H6 2 ION SOURCE FOCUSSING I MPASS s ECTRO- I I V METER TINVENTORS SPEC|MEN CHRISTIAN A. ANDERSEN HENRY J. RODEN FIG. 3 BYATTORNEY BlRlEF SUMMARY This invention relates to a novel method of ionbombardment enabling the achievement of improved and heretoforeunachievable results in processes that include the step of bombarding amaterial with charged particles.

Heretofore in ion bombardment work, either for spectrochemical analysisby secondary ion emission, or for other purposes such as, for example,the making of solid state electronic devices by ion implantation, thebombarding ions have been selected from among those having a positivecharge. Extremely useful results have been achieved, especially in massspectro-chemical analysis. However, it has not heretofore been possibleto analyze insulating materials by this method with a reasonable degreeof reliability. The surfaces of insulating and poorly conductingmaterials appear to become electrically charged, not only by reason ofthe charges delivered by the impinging ions, but also by the departureof secondary electrons driven from the surfaces by the ions. After arelatively brief bombardment, the surfaces of many insulators becomecharged to the point where the bombarding beam is diffused, or repelled,or in some other way adversely affected so that it is no longer possibleeven to detect the emission of secondary ions.

It has previously been suggested that this effect could be overcome byevaporating a grid of conductive material on the surface to be bombardedso that the charges delivered to the surface by the bombarding ions andcreated thereon by the departure of the secondary electrons could leakoff, having to travel only a short distance along the insulatingsurface. Another previous suggestion was to place a source of electronsclose to the surface of the specimen under bombardment and biasednegatively relative to the specimen so that electrons from the sourcewould be drawn to the specimen to neutralize the positive charge. Theseexpedients have been found to be of very little practical use in mostinstances.

Briefly, in accordance with the present invention, it has been foundthat the hereinabove described problems may be substantially completelyovercome, and excellent results achieved if the specimen is bombardedwith negative ions in place of the previously used positive ions.

The theory on which the invention is based is not understood, but isbelieved probably to be related to the known phenomenon that allmaterials release secondary electrons when subjected to ion bombardment.In the case of materials that are electrically insulating, thestatistical yield of secondary electrons is greater than the number ofimpinging ions when the bombarding ion energies are in the rangeordinarily used in microanalysis work. The electrons also greatlyoutnumber the yield of sputtered ions, and the charging effect on thesurface of the material being bombarded seems to be primarily determinedby charges delivered by the impinging ions and charges taken away by thesecondary electrons. The secondary electrons seem to be usually ofrelatively low energy so that, as the surface under bombardment startsto charge in the positive direction due to the departure of secondaryelectrons in greater numbers than the arriving negative ions, enough ofthe secondary electrons are electrostatically attracted back to thesurface to establish an equilibrium condition. The small positivepotential at the point of impact of the bombarding ions is insignificantcompared to the acceleration imparted to the bombarding ions, which isusually in the range of about l,000 to 20,000 electron volts.

Thus, by the use of negative ions, the heretofore disastrous effectsproduced by electrostatic charge accumulation on the surface of thespecimen are avoided. The technique has been found to be extremelyuseful in the operation of an ion microprobe of the type described andclaimed in the copending application of ll-llelmut I. Liebl, Ser. No.494,388, filed Oct. 1 l, 1965, entitled Ion Microprobe" and assigned tothe present assignee. It is also thought that it will be found highlybeneficial for sputtering in general, and in other applications also,especially in connection with ion implantation. For example, a growingdegree of interest is currently evident in industry in the use of ionimplantation methods for making solid state electronic devices. The useof negatively charged ions in accordance with the invention is expectedto facilitate the achievement of higher concentrations of implanted ionsinto more precisely defined regions, relative to implantation bybombardment with positive ions.

DETAILED DESCRIPTHON The invention will now be described in greaterdetail in connection with the accompanying drawing, wherein:

FIG. l is a chart illustrating the secondary yield of Al ions from analuminum specimen in response to bombardment with positive oxygen ions;

FIG. 2 is a chart on a comparable scale of the yield of Al ions from analumina specimen in response to bombardment by negatively charged oxygenions; and

FIG. 3 is a schematic block diagram of apparatus according to theinvention.

Insofar as is presently known, an impinging ion produces the same effectin a material regardless of its charge in respect of sputtering ofmaterial from the specimen and the release of secondary electrons. Theimpinging ion is simply a submicroscopic bullet delivering energy inaccordance with its velocity and mass. It appears to become electricallydischarged as it approaches or meets the surface. The polarity of thecharge carried by the impinging ions, therefore, appears to beimmaterial in the bombardment of electrically conductive specimens.Whether the ions are electrically negative or positive, the sputteringresults are substantially identical for similar beam currents andparticle energies. Either positive or negative ions may be chosen on thebasis of collateral considerations such as the characteristics of theion source.

When electrically insulating specimens are to be bombarded, however, thecharges carried by the bombarding ions and deposited upon the surfacesof the specimens are trapped there, and the selection of ions of theproper charge becomes a matter of vital importance.

FlG. ll illustrates the output of secondary positive aluminum ions ofatomic weight 27 sputtered from an aluminum specimen in response tobombardment by a beam of 0 ions (nascent oxygen, atomic weight l6) in anion microprobe of the type described in the hereinabove identifiedcopending application of Helmut I. Liebl. It is seen that within a fewseconds of the start of the bombardment, the output of sputtered ionsreaches a relatively high and stable value.

The use of oxygen ions in place of the more commonly used chemicallyinert ions such as argon ions is described and claimed in the copendingpatent application of Christian A. Andersen and l'lelmut J. Liebl, Ser.No. 678,840, filed Oct. 30, 1967, entitled Analysis by Bombardment withOxygen Ions, now abandoned, and the continuation-in-part thereof, Ser.No. 753,822, filed Jul. 12, 1968, entitled, Analysis by Bombardment withChemically Reactive Ions."

lFlG. 2 is a chart on the same scale as the chart of HG. ll showing theemission of secondary Al ions from a specimen of relatively pure alumina(A1 0 under bombardment in the same instrument by a beam of 0 ions. inboth cases, the current in the ion beam was about 3X10 amperes, theaccelerating potential was about 8 kilovolts, and the ion beam wasdefocused to cover an area of between about 20 and 50 1?.

it is seen that the output of aluminum ions from the alumina shows acharacteristic very similar to that obtained in the case of metallicaluminum. The output of sputtered ions rises rapidly to about the samerelatively high and stable value. The selection of negative ions toconstitute the primary beam successfully alleviates the problemsheretofore encountered due to building of an electrical charge on thesurface of the insulatOt'.

The negative ions may be produced in any desired way. It is known, forexample, that by properly positioning the outlet orifice of an ionsource of the type known as a duoplasmatron, negative ions can be drawnfrom it in relatively large numbers. No claim is made in thisapplication relative to the negative ions per se, or to the means forproducing them. The invention is directed chiefly to the concept ofusing negative ions for bombarding materials, especially for bombardingelectrically insulating materials, for any desired purpose. Also, sincethe nature of the matter sputtered from the bombarded specimen appearsto be independent of the electrical polarity of the bombarding ions, thepractice of the invention in spectrochemical work is not is any waylimited in respect of the selection of sputtered ions for analysis, orof the methods used for analyzing the sputtered ions or other materials.

FIG. 3 schematically represents the ion microprobe described in thehereinabove-identified box labeled FOCUS- lNG indicates a system oflenses for producing an ionic image of the source upon the surface ofthe specimen.

The secondary electrons are ejected from the bombarded surface atrelatively low velocities, and it is desirable to provide a collectorelectrode (H6. 3) fairly close to the surface to ensure against theaccumulation of an excessively large negative space charge adjacent tothe surface. Electrodes suitable for this purpose are normally presentin all ion bombardment instruments equipped for mass spectrometricanalysis of secondary ions, and instruments of this type ordinarily needno special modification for collecting the secondary electrons. in othertypes of ion bombardment apparatus, it may be desirable to add a simple,positively biased electrode spaced within an inch or so of the specimento be bombarded.

The spacing and bias are not critical.

We claim:

1. Method of implanting particles of atomic dimension in a specimen ofan electrically insulating material comprising the step of bombardingthe specimen with negatively charged ions, imparting energy to the ionsat a value selected to cause them to drive secondary electrons out ofthe specimen to remove negative charges at a rate to compensate fullyfor the negative charges carried to the surface of the specimen by theions thereby to avoid the accumulation of an excessive negativeelectrical charge on the surface of the specimen.

2. Method of sputtering an electrically insulating material comprisingbombarding the material with negatively charged ions to cause particlesof the material to be ejected from it, imparting energy to the ions at avalue selected to cause them to drive secondary electrons out of thespecimen to remove negative charges carried to the surface of thespecimen by the ions thereby to avoidthe accumulation of an excessivenegative electrical charge on the surface of the specimen.

3. Method of analyzing an electrically insulating material comprisingthe steps of bombarding a specimen of the material with negativelycharged ions with enough energy to sputter secondary ions from thematerial, imparting energy to the ions at a value selected to cause themto drive secondary electrons out of the specimen to remove negativecharges at a rate to compensate fully for the negative charges carriedto the surface of the specimen by the ions thereby to avoid theaccumulation of an excessive negative electrical charge on the surfaceof the specimen, and mass spectrometrically analyzing the sputteredions.

2. Method of sputtering an electrically insulating material comprisingbombarding the material with negatively charged ions to cause particlesof the material to be ejected from it, imparting enerGy to the ions at avalue selected to cause them to drive secondary electrons out of thespecimen to remove negative charges carried to the surface of thespecimen by the ions thereby to avoid the accumulation of an excessivenegative electrical charge on the surface of the specimen.
 3. Method ofanalyzing an electrically insulating material comprising the steps ofbombarding a specimen of the material with negatively charged ions withenough energy to sputter secondary ions from the material, impartingenergy to the ions at a value selected to cause them to drive secondaryelectrons out of the specimen to remove negative charges at a rate tocompensate fully for the negative charges carried to the surface of thespecimen by the ions thereby to avoid the accumulation of an excessivenegative electrical charge on the surface of the specimen, and massspectrometrically analyzing the sputtered ions.